The present invention relates to phase shifters in general, and more particularly, to a phase shifter circuit element including a matched pair of coupled transmission lines which cooperate with a quadrature coupler branch network and a pair of symmetric reflecting pin diode terminations to effect phase shift switching speeds on the order of a few nanoseconds, the coupling of said transmission line pairs being selectable to offer a variety of phase shift values.
In some high resolution mapping type radar systems, digitally controlled phase shifters are included in the sampling linearizers of the linear FM (chirp) waveform generator of the radar system to achieve the range resolution required. For a more detailed description of a sampling linearizer utilizing a phase shifter, reference is hereby made to the U.S. patent application Ser. No. 935,240, now U.S. Pat. No. 4,160,956 which is filed concurrently herewith and assigned to the same assignee as the present application. The phase shifting elements which are responsive to the digital control bits of the sampling linearizer are normally required to operate to shift the phase of a signal conducted therethrough at very high switching speeds. Typical desired sampling times are on the order of 25 nanoseconds or less in some instances. To achieve reasonable steady state conditions over the sampling interval, switching speeds on the order of a few nanoseconds may be required in some cases.
At the present time, most applications of a phase shifting circuit element are best met using a three dB quadrature coupler 10 with symmetric reflecting diode terminations 12 as depicted in FIG. 1. The incident voltage V.sub.G of the input RF signal at port 1 of the coupler 10 is transmitted to the phase splitting ports 2 and 3 of the coupler 10 with voltages of equal magnitude but with a phase difference of 90.degree. . The voltages refected at the terminations (see 14 and 16) remain 90.degree. out of phase since the terminations are identical. The reflected voltages are each transmitted through the coupler 10 where they are recombined at an output port 4. Since the output voltage V.sub.T at output port 4 consists only of two reflected waves, the phase shift at the output is equal to the phase shift provided by the reflective terminations.
The phase shift achieved at the reflective terminations depends on the particular characteristics of the pin dioder 18 and 20 as well as the nature of the reactive matching networks 22 and 24. To a first approximation, the pin diode acts as a switch which is alternated between a low impedance and a high impedance state when operated respectively between forward and reverse bias states as governed by the voltage supplied at point 26. An ideal diode would alternately look like a short and open circuit, therefore shifting phase by 180.degree. . Other values of phase shifts are achieved by reactively trimming the reactive networks 22 and 24.
There are a number of ways in which the reactive trimming may be achieved. When dealing with microwave frequencies, distributed circuit elements are most commonly used. Typical distributed elements are shorted and open circuited stubs and/or sections of transmission line. One commonly used matching network (and perhaps the simplest) is the network shown in FIG. 2. A one eighth-wavelength section of line 30, for example, transforms an ideal diode 32 (whose phase shift is 180.degree. ) to an impedance of .+-./JZ.sub.T which produces a phase shift of 2 tan.sup.-l (Z.sub.T /Z.sub.O). The particular choice of reactive trimming depends on the phase shift desired, the frequency bandwidth and the range of realizable impedances.
A multi-bit phase shifter may be obtained by cascading several of these known phase shifter networks as shown in FIG. 3. Each phase shifting element is substantially identical except for the reactive matching networks 34 and 36 which determine the particular phase shift of each phase shifting element. The inter-section capacitors 38 are used to couple the RF signals from one element to the next, and to decouple each phase shifting element so that each may be switched independently from the other.
The general problem of switching a reflection-type diode phase shifter is to provide a circuit which allows for applying the appropriate bias to the diodes without affecting the RF transmission properties of the phase shifter network. FIG. 4 illustrates a general block diagram schematic of a typical phase shifter element. A low pass (or RF band stop) filter 40 located between the diode 42 and the diode driver 44 must allow the bias voltage/current I.sub.b to be applied to the diode 42 and act as a very high impedance to the RF frequencies. A high pass (or RF bandpass) network 46 must allow for the RF signals to reach the diode 42 without affecting the phase shift properties of the phase shifting circuit and to isolate the bias signals generated by the drivers 44 from the rest of the RF network. At low switching rates, a series capacitor in the RF line is usually sufficient. The capacitance is chosen high enough to provide a very low impedance to the RF frequencies and yet sufficient enough to block the diode bias signals.
At high switching rates, where the frequencies of the switching pulse approach the RF or carrier frequency, as that needed for the sampling linearizer of the FM generation system of a high resolution mapping radar, for example, simple blocking capacitors no longer suffice. In order that the driver pulse be allowed to rise quickly, extraneous capacitances in the form of RF bypass elements must not be excessive. This problem may be solved by providing more sophisticated filtering with sharper cut-off properties or by utilizing the high impedance properties necessary at the switching frequency as part of the RF reactive phase matching. The phase shifting circuit element disclosed hereinbelow offers these characteristics.